1. Field of the Invention
This invention relates to a television camera containing therein a color resolving optical system having a bias illuminating device.
2. Description of the Prior Art
For example, in an ordinary color television image pick-up device, a color resolving optical system is disposed at the back focal portion of the objective lens and the light beam from an object to be photographed is resolved into three light beams of blue, green and red wavelength ranges by the color resolving optical system and these light beams are caused to enter the light-receiving surfaces of the image pick-up tubes of corresponding color channels, whereby blue, green and red images of the object are formed on the light-receiving surfaces. As the color resolving optical system, use is usually made of a plurality of prisms or glass plates each having deposited thereon by evaporation a multi-layer interference thin film reflecting a light of a particular wavelength range and transmitting therethrough lights of the other wavelength ranges, i.e., a so-called dichroic film, the prisms or glass plates being disposed obliquely with respect to the optical axis of the objective lens. For example, by successively disposing a dichroic film reflecting the blue wavelength light and transmitting therethrough the other wavelength lights and a dichroic film reflecting the red wavelength light and transmitting therethrough the other wavelength lights, and causing the lights reflected by and transmitted through the dichroic films to enter the light-receiving surfaces of the image pickup tubes, the light beam for the object can be resolved into the aforementioned light beams of three colors.
Popular materials of the light-receiving surfaces of the image pick-up tubes include lead oxide (PbO), arsenic selenide (AsSe.sub.2), cadmium selenide (CdSe), etc., and in a television camera adopting these, it is effective to normally irradiate the image pick-up tube surfaces with a bias illuminating light of low illumination and having a uniform wavelength distribution in order to improve the response speed and afterimage characteristics of the image pick-up tubes during the rising response interval thereof.
FIG. 1B of the accompanying drawings depicts a structure in which a bias light is applied from a portion of a color resolving prism, and this Figure shows the form of the color resolving prism of FIG. 1A of the accompanying drawings as sectioned on the optical axis, which is also the optical axis of objective lens 1 in both figures. In FIG. 1B, reference numeral 2 designates the color resolving prism provided with an internal reflecting surface 4 and cut-away portions 4', 4" at the corners thereof. The reflecting surface 4, as shown in FIG. 1A, is a diffusing-reflecting surface extending about the optical axis while, on the other hand, the surfaces 4' and 4" forming the cut-away portions are diffusing-transmitting surfaces. Designated by 5 is an illuminating light source which may be, for example, a tungsten lamp. Reference character 6a designates a color temperature converting filter, reference character 6b denotes a quantity of light adjusting filter, and reference numeral 7 designates a reflecting mirror provided with a bent surface.
The light beam emitted from the illuminating light source 5 passes through the two filters 6a and 6b and is reflected by the reflecting mirror 7, and part of the reflected light passes through the cutaway surface 4' and is reflected by or transmitted through the dichroic film, whereafter it enters the image pick-up surface 3' of each image pick-up tube 3. The other part of the reflected light passes through the cut-away surface 4" and is reflected by the diffusing-reflecting surface 4, and then likewise enters the image pick-up surface 3'. It is for the purpose of correcting the non-uniformity of the illumination that the image pick-up surface is irradiated from above and below the optical axis. The filters used in the above-described bias illuminating structure are disposed to supply bias lights of moderate proportions to the image pick-up tubes 3B, 3G and 3R, but where the bias light supplied to a certain image pick-up tube is insufficient, a difference is created in the afterimage reducing effect, which in turn leads to the inconvenience that the afterimage is tinged with colors. Further, the lamp used as the illuminating light source, the color temperature converting filter, the color resolving optical system, etc. have irregularities of spectral distribution due to manufacturing errors, or variations in the lamp with time and variations in spectral distribution as a result of interchanging components of the system can be introduced and therefore, it is very difficult to adjust or balance the bias illumination into an optimum condition.
A method for solving this problem is to dispose individual bias illuminating devices immediately in front of the image pick-up tubes 3B, 3G and 3R, but such method suffers from the difficulty that an extra space suitable for applying a light beam or an extra space in which the illuminating devices are disposed must be prepared or the three illuminating devices must be separately assembled and adjusted.
Even if such difficulty is solved, the dichroic film for color resolution is inclined with respect to the image pick-up optical axis and this leads to color shading. This phenomenon will now be described with an illustration of a bias illuminating device using clad rods. The clad rods 101 shown in FIG. 2A of the accompanying drawings each comprise a glass pillar of high refractive index covered with glass of low refractive index so that light introduced thereinto through one end surface thereof propagates therethrough while repeating total reflection on the boundary surface. Two clad rods 101, 101 are disposed on the upper and lower portions of the front of a color resolving optical system 102, and a portion of each of these clad rods 101 is shaved into a rough surface, which is painted in white to form a diffusing surface 103. The lights of bias lamps 104 having a flat wavelength characteristic are caused to enter these clad rods 101, 101 through the end surfaces thereof and enter the color resolving optical system 102 through the diffusing surfaces 103 as bias lights. The color resolving optical system 102, as shown in FIG. 2B, comprises first, second and third prisms 105, 106 and 107 connected together in the named order from the front, with the rear surfaces of the first and second prisms 105 and 106 being disposed obliquely with respect to the optical axis O of the objective lens, and dichroic films 151 and 161 are provided on these surfaces.
Accordingly, the bias lights emitted from the bias lamps 104 and diffused by the diffusing surfaces 103 enter the color resolving optical system 102. In the color resolving optical system 102, the bias lights first enter the entrance surface 152 of the first prism 105 and, of these incident lights, only the blue wavelength range light is reflected by the dichroic film 151, and this blue wavelength range light is further totally reflected by the entrance surface 152 and enters the light-receiving surface 110b of the image pick-up tube 109b through a trimming filter 108b provided on the exit surface 153 to adjust the condition of the spectral distribution on the light-receiving surface 110b. Also, of the bias lights transmitted through the dichroic film 151, from which the blue wavelength range light has been eliminated, only the red wavelength range light is reflected by the dichroic film 161 of the second prism 106 and is further totally reflected by the boundary surface 162 at the parallel air gap provided between the first prism 105 and the second prism 106, whereafter it passes through the trimming filter 108r on the exit surface 163 and enters the light-receiving surface 110r of the image pick-up tube 109r. The bias light which has been transmitted through the dichroic film 161 and from which the blue wavelength range light and the red wavelength range light have been eliminated is green wavelength range light, and this light passes through the third prism 107 and enters the light-receiving surface 110g of the image pick-up tube 109g through the trimming filter 108g.
In the above-described apparatus of the prior art, observing the quantity of light distribution of the bias lights entering the light-receiving surfaces 110b, 110r and 110g, respectively, of the image pickup tubes 109b, 109r and 109g, the relative quantity of light increases or decreases in a certain direction and that direction differs along the light-receiving surfaces 110b, 110r and 110g. This is because the wavelength selecting characteristic of the dichroic films 151 and 161 depends on the angle of incidence of the light onto these films and the wavelength selecting characteristic varies with a variation in the angle of incidence, and this may cause color irregularity, i.e., color shading, to occur on the screen of the television when use is made of a television camera containing therein a color resolving optical system having a dichroic film.
The color shading will hereinafter be described in detail. In the diffusing surface 103 shown in FIG. 2B of the accompanying drawings, a point on the optical axis O of the objective lens is defined as P, a light ray emergent from the point P so as to pass along the optical axis O is defined as c, and light rays emergent from the point P toward the ends of the light-receiving surfaces 110b, 110r and 110g are defined as u and l. The incident lights of the light rays u, c and 1 which enter the light-receiving surfaces 110b, 110r and 110g are respectively defined as LBu, LRu, LGu; LBc, LRc, LGc; and LBl, LRl, LGl, and the points of incidence of these lights are defined as Bu, Ru, Gu; Bc, Rc, Gc; and Bl, Rl, Gl. If the light rays u and 1 have an inclination of angle .theta. with respect to the optical axis O and the angles of incidence of the light ray c onto the dichroic films 151 are 161 are .alpha. and .beta., respectively, then the angles of incidence the light ray u onto the dichroic films 151 and 161 are (.alpha.-.theta.) and (.beta.+.theta.) and the angles of incidence of the light ray l onto the dichroic films 151 and 161 are (.alpha.+.theta.) and (.beta.-.theta.), and between the angles of incidence of the light rays, u, c and l onto the dichroic films 151 and 161, there are the relations that (.alpha.+.theta.)&gt;.alpha.&gt;(.alpha.-.theta.)&gt;0 and (.beta.+.theta.)&gt;.beta.&gt;(.beta.-.theta.)&gt;0.
FIGS. 3A and 3B of the accompanying drawings are graphs showing the transmitting characteristics of the dichroic films 151 and 161. In these graphs, the ordinate represents transmittivity, the abscissa represents wavelength, the dotted line indicates the transmittivity curve of the light ray u, the solid line indicates the transmittivity curve of the light ray c and the dot-and-dash line indicates the transmittivity curve of the light ray l. As is apparent in these graphs, the light rays l and u, whose angles of incidence onto the dichroic films 151 and 161, respectively, are relatively greater than those of light rays u and l, respectively, tend to be transmitted in relatively shorter wavelengths thereby, while the light rays u and l, whose angles of incidence onto the dichroic films 151 and 161, respectively, are relatively smaller than those of light rays l and u, respectively, tend to be transmitted in relatively longer wavelengths thereby, since the transmitting characteristics of the dichroic filsm 151 and 161 are such that they tend to transmit toward the short wavelength side as the angle of incidence becomes greater. Also, generally, relative to a plane orthogonal to the optical axis O of the objective lens in the color resolving optical system 102, the angle of incidence .alpha. onto the dichroic film 151 of the first prism 105 is in the relation that 23.degree.&lt;.alpha.&lt;30.degree. and the angle of incidence .beta. onto the dichroic film 161 of the second prism 106 is in the relation that 10.degree.&lt;.beta.&lt;15.degree. and thus, the angle of incidence .alpha. onto the dichroic film 151 is greater than the angle of incidence .beta. onto the dichroic film 161 and therefore, as shown in FIGS. 3A and 3B, the fluctuation of the transmittivity by the wavelength range of the light ray is greater for the first dichroic film 151 than for the second dichroic film 161.
FIGS. 4A, 4B and 4C of the accompanying drawings show the transmitting characteristics of trimming filters 108b, 108g and 108r. In these Figures, the ordinate represents transmittivity and the abscissa represents wavelength. Considering the spectral distributions on the light-receiving surfaces 110b, 110r and 110g on the basis of FIGS. 3 and 4, there are obtained the characteristics shown in FIGS. 5A, 5B and 5C of the accompanying drawings. In FIGS. 5A-5C, the ordinate represents the quantity of light which has reached the light-receiving surface 110b, 110g, 110r, the abscissa represents wavelength, the dotted line indicates the spectral distribution curve of the light ray u on each light-receiving surface 110b, 110g, 110r, the solid line indicates the spectral distribution curve of the light ray c on each light-receiving surface, and the dot-and-dash line indicates the spectral distribution curve of the light ray l on each light-receiving surface. Also, the relative quantity of light at each position of the light-receiving surfaces 110b, 110g and 110r is shown by solid line in FIGS. 6A-6C of the accompanying drawings. According to these Figures, when the bias lights have reached the light-receiving surfaces 110b, 110r and 110g through the prisms 105, 106 and 107, the quantity of light differs depending on the position of incidence thereof and thus, a light-and-dark difference occurs. When the television screen to which these quantities of light are put out after being converted into electrical signals is viewed, there occurs a phenomenon that colors differ at the opposite ends of the screen, and such phenomenon is called the color shading of bias illumination. Also, as described above, the blue wavelength range light and the green wavelength range light near the blue wavelength range create great color shading due to the difference between the angle of incidence of light ray onto the first dichroic film 151 and the angle of incidence of light ray onto the second dichroic film 161, while the red wavelength range light only creates relatively small color shading. Such color shading is not a phenomenon which appears only in the above-described apparatus of the prior art, but is a disadvantage common to all television cameras of the type in which a substantially white bias light source is disposed in front of a color resolving optical system and the bias light is resolved into a plurality of wavelength range lights by the use of the dichroic films in the color resolving optical system, whereafter the lights are directed to the light-receiving surfaces of respective image pick-up tubes. Accordingly, where images of high quality are required, it is desirable to eliminate also the color shading resulting from the difference in inclination between the light rays incident on the dichroic films.